The present invention relates to transformers for converting three-phase power to nine-phase power, and more particularly to transformers for providing reduced harmonics on the AC and minimizing ripple on the DC side of an AC to DC rectifier.
Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion, which, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three phase and therefore three phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. As with most hardware intensive configurations cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases, which are rectified by the rectifier. These AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase, harmonics and ripple are reduced appreciably. Where AC harmonic restrictions are placed on rectifier systems such restrictions are often satisfied by employing an 18-pulse rectifier that requires a nine-phase source of AC power. As the global standard for AC power distribution is three phase, 18-pulse rectifiers require three-to-nine phase power converters between utility supply lines and rectifier switches.
Isolation transformers for converting three-phase AC power to nine-phase AC power are known in the art but have several shortcomings. First isolation transformers must be rated for the full power required. Second, isolation transformers are typically relatively large as separate primary and secondary windings are required for isolation purposes.
Where isolation between a utility supply and a rectifier is not required, employing an autotransformer including a plurality of series and common windings may advantageously reduce the size and weight of a three-to-nine phase converter which consists of an autotransformer and a rectifier unit. Exemplary three-to-nine phase autotransformers are described in U.S. Pat. Nos. 4,876,634 (the "'634 patent"); U.S. Pat. No. 5,124,904 (the "'904 patent"); U.S. Pat. No. 5,619,407 (the "'407 patent"); and U.S. Pat. No. 5,455,759 (the "'759 patent"), each of which is incorporated herein for the purpose of describing the prior art.
The '634 patent teaches the general concept of providing three phase autotransformer coils in a plurality of series connected windings which are arranged to form a hexagon. Three phase AC input lines are linked to three input nodes and nine output nodes provide voltages to three rectifier bridges. Phase shift between the output voltages is accomplished by providing long and short windings between the input nodes and the output nodes. Importantly, the '634 patent teaches that, for each autotransformer input phase, the phase shift between three corresponding output voltages should be 20 degrees and accomplishes 20 degree phase shift by providing short windings between each two adjacent output nodes corresponding to the same input phase. Long windings are provided between adjacent output nodes corresponding to different input phases. In the '634 patent the nine output voltages are provided to three separate six-pulse bridges.
Unfortunately, there are at least two problems with the 18-pulse autotransformer described in the '634 patent (hereinafter the '634 topology). First, there is an inherent impedance mismatch in the '634 topology which results in looping currents among the three bridges and which requires additional hardware to correct. For example, when the outputs and inputs to the '634 18-pulse autotransformer are linked to provide unity gain one of the three bridges is fed directly from the input power source while the other two bridges are fed through transformer windings which each are characterized by a certain amount of leakage inductance. This means that there are different impedances for each of the bridges and the different impedances cause disparate DC output voltages and hence looping currents among the bridges. A similar impedance disparity results when the '634 patent 18-pulse autotransformer is linked for step-down transformation.
The '634 topology attempts to use two inter-phase transformers to reduce the looping currents. As an initial matter Applicant believes the inter-phase transformers provided in the '634 topology are erroneously specified and that six, not two, inter-phase transformers would be required to reduce the looping currents. While six inter-phase transformers can be provided inter-phase transformers are required to carry DC bus currents and therefore are relatively bulky and increase system size appreciably. In addition the six inter-phase transformers are relatively expensive and increase system costs.
Second, the '634 topology would result in current sharing problems among the three bridges due to enclosed electrical circuits formed by the multi-phase shift bridges. The current sharing problems are exacerbated when AC line harmonics occur as different source harmonics substantially change bridge current sharing. Because AC line harmonics are often irregular and unpredictable it is impossible to balance the impedance mismatch via addition of resistance elements. While the inter-phase transformers are may ease current harmonics to the power source, the inter-phase transformers are not effective as a solution for the current sharing problem.
Because of the current sharing problem described above all three bridges in the '634 topology have to be capable of handling over-rated current conditions as high as 150% of the current level required to be handled if the bridges were able to share current equally. This is because form time to time each bridge is forced to operate close to its rated current level while the other bridges only operate at 50% of their rated level. This drastic current difference among bridges also forces the windings of the '634 topology to carry appreciably disparate current magnitudes. For this reason, in addition to the bridges having high current ratings, the autotransformer also must be rated to handle high current value and therefore results in inefficient material utilization.
One solution to the looping and sharing current problems associated with the '634 topology is to provide an autotransformer that equally spaces output voltages in phase. For example, where there are nine outputs, the outputs can be phase shifted from each other by 40 degrees each. In the '407 patent this is accomplished by providing an autotransformer having three coils, each coil having a plurality of serial windings and a plurality of stub windings. The serial windings form a delta and the stub windings are magnetically coupled with the serial windings from the same coil. Three terminals are provided ad the apices of the delta and the three phase AC inputs are linked to the apex terminals. A plurality of direct outputs is interposed between respective serial windings and a plurality of indirect outputs is electrically connected with the second ends of the stub windings. The windings are chosen such that the voltage magnitudes of the direct and indirect outputs are identical. Other autotransformer topologies which include stub windings are described in the '904 patent and the '759 patent.
While staggering the transformer outputs by 40 degrees essentially eliminates the looping and sharing current problems identified above, the stub winding requirement in each of the '407, '904 and '759 patents renders those solutions wasteful of winding and core material.
While all of the prior art autotransformers identified above teach topologies that cause step-down transformation, some DC loads require DC voltage magnitudes that are higher than an AC input voltage magnitude. In these cases the step-down transformers described above are not suitable and instead a step-up transformer is necessary.
Despite the relatively large size of isolation transformers some applications require isolated primary and secondary windings. In the isolated transformer topologies many of the same design concerns have to be considered. For example, isolation transformers should be designed so as to minimize input current harmonics, minimize DC bus voltage ripple, eliminate bus current sharing problems, reduce overall transformer size and minimize required materials thereby reducing costs.
Thus, it would be advantageous to have a three-to-nine phase transformer that did not cause looping and sharing current problems and that is relatively inexpensive to construct. In the case of an autotransformer it would be advantageous if the transformer could be used either as a unity gain or a step-up transformer.